Movable contact for electric current



' R. T. ERBAN MOVABLE CONTACT FOR ELECTRIC CURRENT Nov. 10, 1-9 70 Filed July 12. '1965 3 Sheets-Sheet 1 INVENTOR.

R. T. ERBAN MOVABLE CONTACT FOR ELECTRIC CURRENT 3 Sheets-Sheet 2 Filed Julia 12, 1965 W/l/l INVENTOR.

Nov. 10; 1970 R. T. ERBAN MOVABLE CONTACT FOR ELECTRIC CURRENT 3 Sheets-Sheet 5 Filed July 12, 1965 lrrlinnl United States Patent Filed July 12, 1965, Ser. No. 471,316 Int. Cl. H01c /04 US. Cl. 338-202 5 Claims ABSTRACT OF THE DISCLOSURE A movable contact device such as used in potentiometers, commutators and slip-rings wherein contact with a moving contact roller is established at polar regions thereof to minimize friction, wear and electrical noise and wherein the surfaces of the roller and of the cooperating contact members are shaped to provide a rolling-contactarea which, under operating pressure, is elongated transversely to the direction of travel of the roller to reduce the contact pressure per unit of area in order to keep the mechanical stress inside the contact material within the limits applicable to said material for prevention of fatigue by repetitive stress cycling.

This invention relates to devices for transmitting electrical current or signals between conductors which are movable relatively to each other, and where operating conditions require a minimum of distortion of the transmitted signals through so-called electrical noise.

Contacts of this kind are found on potentiometers, particularly for use with analog computers, on digital encoders, in connection with movable highly sensitive transducers, etc. Known constructions of movable contacts of the dry type, that is, without the help of conducting liquids or mercury wetted surfaces, belong to two basic forms: those using a gliding friction contact between a resilient brush and a rigid conductor; and those which use a conductive roller or ball as a movable transmitting element in rolling contact with the relatively stationary conductor. Both of these systems have very serious defects when high quality performance is required. The operation of a sliding brush requires substantial forces to overcome friction in all cases where the contact pressure is great enough to guarantee satisfactory electrical performance; furthermore, the friction forces cause wear which in most cases takes place non-uniformly over the length of the resistance element of a potentiometer, thereby progressively destroying its original linearity. Movable contacts with balls or rollers instead of sliding brushes have been found even less satisfactory in many respects than sliding contacts, particularly in respect to signal distortion caused by electrical noise.

The reasons for these failures will become fully apparent later in this specification where a detailed physical analysis of the new structure will clearly point out the basic distinction of this invention from known devices. The main object of this invention is a movable contact which prevents the occurrence of the defects experienced with contact devices of known design, while at the same time securing and maintaining a movable electrical contact having a non-varying, or constant, electrical contact potential of relatively low value so that the electrical noise incident to movements of the contact is practically zero; another object of the invention is to provide a movable contact that has a long operating life and which requires a minimum of power for its operation. Other objects of the invention will be pointed out as they become apparent in the following disclosure.

The present invention achieves the above outlined objects by incorporating two basically new structures in the general solution of a rolling contact. The first of the new structures concerns the moving contact between the roller member and the surface of the cooperating conductor, which may be a slip ring, a commutator or the resistance element of a potentiometer. Reliable operation of the device, long life and low electrical noise of the transmitted signals require a close control 'of the mechanical stresses throughout the contact area and the material adjacent to it. This is obtained by a predetermined geometric configuration of the two contact surfaces which meet in the rolling contact. The second of the new structures concerns the means for transferring the current which has passed into the roller via the moving contact, from the roller to another stationary conductor. This is obtained by a new type of movable contact which hereafter is termed a spinning contact because of the specific kind of relative motion which takes place between the two contacting surfaces.

My invention is fully disclosed, together with the essential parts of newly discovered physical phenomena and their analysis, in the following specification and illustrated by way of example in the accompanying drawings in which:

FIG. 1 is a top view of a typical potentiometer, schematically showing the position of the new contact device;

FIG. 2 is an elevated view of a section of FIG. 1

along the plane indicated by the arrows S-S;

FIG. 3 shows in enlarged scale a portion of the view of FIG. 2 with all details of the new structure;

FIG. 4 is a partly sectional side view of FIG. 3, as indicated by the arrows U-U;

FIG. 5 illustrates a modification of the structure of FIG. 4;

FIGS. 6 and 7 are schematical diagrams useful in analysing the positions and the effects of physical forces arising during the operation of the structures of FIGS. 3, 4, and 5, respectively;

FIG. 8 illustrates an elevated partial section of a modification of the structure of FIG. 3;

FIG. 9 is a top view of a section of 'FIG. 8;

FIG. 10 is a side view of FIG. 8, as indicated by arrows W-W;

FIG. 11 is a side view similar to FIG. 10, illustrating a further modification of FIG. 8;

FIGS. 12, 13 and 14 are schematical diagrams showing in a large scale the essential contact elements of FIG. 3 and the pertinent physical entities controlling the geometry of the contact area;

FIG. 15 is an elevated sectional view of two movable contacts positioned on adjacent commutator segments and slip-ring respectively, in which non-modified balls are employed as rolling contact elements;

FIGS. 16 and 17 are side views of the two variants of ball support as shown in FIG. 15;

FIG. 18 is a top view of the adjacent commutator and slip-ring contact members of FIG. 15, showing schematically the rolling tracks of the balls of FIG. 15;

FIG. 19 is an elevated section of a modification of the ball support shown in FIG. 16 for the ball riding the commutator;

FIG. 20 is a side view of this arrangement;

FIG. 21 is a top view of the ball track of FIG. 18 in enlarged scale and illustrates the elliptical contact area;

FIG. 22 is a diagrammatical section through the polecontact of a rotating contact element, illustrating some of the physical conditions in a spinning contact accord- FIG. 24 is a diagram illustrating the conditions of current flow through a spinning contact as shown in FIGS. 22 and 23,

FIG. 25 is a similar diagram of'the conditions of the current flow through a conventional brush-type contact.

FIGS. 26 and 27 are schematical diagrams illustrating pertinent physical forces acting upon a rotatable ball as shown in FIGS. 19 and 20,

FIG. 28 is a schematical diagram showing the dimensions of the contact area and its displacement as determined by the geometry of the structure, as illustrated in FIG. 27,

FIG. 29 shows a facsimile reproduction of a photomicrograph of a track observed after prolonged operation of a rolling contact device according to this invention,

FIG. 30 shows a facsimile reproduction of a photomicrograph of a track observed after a shorter period of operation of a rolling contact of inferior design, showing clearly the beginning destruction of the ball track, caused by defective rotation of the ball.

FIGS. 1 and 2 illustrate the application of the invention to a potentiometer having a wire-wound resistance element which comprises a core 1 with resistance winding 2; the ends of the winding are fastened at the terminals 10, 10, and the resistance element is carried by the base 3 of insulating material. The base 3 holds the element 1-2 concentric with the hub 7 which provides the bearing for the pivot shaft 6 of the contact arm 5. The bearing between shaft 6 and hub 7 is shown as a plain bearing, but is to be exchanged for an anti-friction bearing 'where a minimum of friction is desired. The contact arm is provided on its outer end with a cavity 4 which is so positioned that its opening sweeps over the upper edge of the wire windings 2 when the arm 5 is turned about its pivot shaft 6. Slidingly supported in the cavity 4 is the roller contact member 9, having a special, partly spherical form of surface; this special surface comprises a generally concave grooved middle portion positioned between the spherical end portions or polar regions. This roller contact member 9 is shown in FIGS. 1 and 2 only in its approximate position and without the means of its support. The accurate position of the contact member 9 and the manner of its support are illustrated in the FIGS. 3, 4 and 5 in an enlarged scale.

In FIG. 3, the core of the resistance element is shown in its cross section 1, supporting the resistance wire windings 2. The rotatable contact member 9 is shown in the shape of a sphere, part of which is replaced by a groove of generally concave toroidal shape; the toroidal surface being concentric with the axis of rotation YY of the roller member 9 and its two polar spherical zones 1717. The groove surface 15 makes rolling contact with the upper portions of the wire windings 2 along the are 16 in a manner and under controlled physical conditions of mechanical and electrical stress, which are within a predetermined range of magnitude to assure reliable operation of the device within the limits of performance contemplated by this invention. The details of these requirements for contact geometry and related physical properties are treated in connection with the FIGS. 12, 13 and 14.

Slideable in the cavity 4 is the cage 14 which by means of a top plate 24 and a spring 25 is subject to a measured force tending to move it towards the upper edge of the wire winding of the resistance element. The cage 14 has ribs 27 which support two shafts 23; upon each of these shafts is rotatably mounted a member 22, shown in the shape of a ball. Only one such ball is seen in FIG. 3, while both are visible in FIG. 4. Both balls bear against the groove surface 15 of the contact member 9; thus the pressure of the spring 25 is transmitted through the balls 22 to the contact member 9 and further to the rolling contact area between surface 15 and the surface of the wire winding 2. The side view of a section through FIG. 3 along a plane markedUU is seen in FIG. 4.

A modification of this structure, with one supporting ball 22, is shown in FIG. 5. While it seems that the structure of FIG. 5 is simpler than the structure of FIG. 4, it must be pointed out that the structure of FIG. 4 has an important advantage over the design of FIG. 5, other than a mere duplication of parts, as will be explained in detail below with respect to FIGS. 6 and 7. It is also to be noted that the position of the ball 22 in FIG. 3 will change only imperceptibly whether it is supposed to represent the structure of FIG. 4 or of FIG. 5; therefore, the same FIG. 3 is shown related to FIG. 4 as well as to FIG. 5.

Inspection of FIG. 4 shows the wall of the cavity 4 and the slideable cage 14 which supports the two balls 22, 22 rotatable upon their individual shafts 23, 23. The rotatable contact member 9 is shown with its smallest cross section 15 of its groove surface 15 contacting the upper edges of the wires 2. The tangent to the upper edges of the wires 2 has been marked as 12. FIG. 6 shows a circle which represents the cross section 15 of FIG. 4, rolls upon the tangent 12. The two support balls 22, 22, shown as circles touching 15, are each transmitting one half of the total spring pressure P to the member 9. Since freely rotatable bodies of circular cross section transmit pressure between each other only in the direction of a line intersecting both centers, it follows that the pressure P causes a radial pressure Q between each of the two bodies 22 and the member 9, where Q is given as P 1/ Q /2 cos alpha This central pressure Q produces a component force I for each of the bodies 22, where J Q sin alpha These forces J of the two bodies 22 are opposed to each other and are transmitted through the surface contacts to the center 0 of the member 9. Since the two components J are of equal size, they compensate each other and hold the center 0 on a predetermined fixed position with respect to the centers of rotation of the bodies 22, 22 respectively. With the center 0 remaining in a fixed position, the member 9 and its contact area with the wire edges 12 also remain in a fixed position marked by the point 28 in FIG. 6. Inspection of FIG. 6 shows that this point 28, which is the center of the contact area between member 9 and the wire tangent 12, will always remain within the plane of symmetry between the two centers of rotation of the bodies 22, 22, even if due to deflection under load the vertical distance between the centers of rotation 23, 23 and the tangent 12 should be subject to slight variations; as long as the plane of symmetry of the centers 23, 23 remains in a fixed position, the center of the contact area 28 will also stay in the same fixed position.

This stability of the position of the contact area between the rotating contact member 9 and the wires of the resistance element is a special quality of the present device and marks a basic difference as well as a great advantage over previous designs where the unavoidable wear of parts causes a lateral displacement of the contact area, and thereby a progressive deterioration of the original accuracy of the device.

Referring again to FIGS. 3 and 4, it shall be shown how the transmission of current from the rolling contact member 9 to the terminal 11 is accomplished in a manner which contributes to one of the main objectives of this invention, that is, a minimum of mechanical force requirement for its operation, combined with a virtual absence of electrical noise. The rotatable member 9 is contacted in relatively small areas at its poles of rotation, FIG. 3, 18, 18, by contact fingers 19, 19, shown in the form of fiat strips that form part of a U-shaped collector member 20. In a preferred form, these fingers are of the same material as the surface of the member 9 at the spherical portions 17, 17; the use of the same material at both sides of the contact reduces possible noise by eliminating thermo-electrical potential differences. The collector member 20 with its two contact fingers 19, 19 is freely floating laterally so that it may follow any motion of the member 9 while maintaining equalized contact pressure for each of the two pole contacts 18, 18; a flexible wire connection 21 provides a continuous circuit from the collector 20 to the terminal 11.

Each of these two spinning contacts 18, 18, provides for the transmission of current, a contact area of definite size, operating under physical conditions basically different from those found in conventional rolling or sliding contacts; therefore, they produce new and important qualities with respect to the mechanical and electrical performance of a current transmitting movable contact. These peculiar novel qualities and some of the physical conditions involved will be more fully described on hand of FIGS. 22, 23, and 24. The most outstanding effects of the spinning contact structure as applied to a potentiometer are a drastic reduction of mechanical torque requirements plus virtual elimination of mechanical wear and of electrical noise from these contacts.

The modification shown in FIG. 5 comprises a contact member 9 which is identical with that of FIGS. 4 and 3, but which is pressed towards its contact with the resistance wires 2 by a single rotatable body 22. The shaft 23 upon which 22 can rotate, is held by the cage 14, which is slideable within the cavity 4, under pressure of the spring 25. The schematic diagram of FIG. 7 illustrates the forces acting upon the rotating member 9. The pressure P is transmitted through the single rotatable body 23 to the member 9 in a direction passing through its center of rotation and through its contact point with the wire surface at 28. Under these conditions, there are no force components parallel to the tangent 12, which, in the structure of FIG. 4, were utilized to stabilize the position of the center of the contact member 9. Therefore, separate means must be provided for maintaining a definite position of the center of member 9 and with it of the contact area between member 9 and the wire surface at 28. To this end, stationary abutments 29, 29 are provided, which contact the surface of member 9 on one side while leaving a minute clearance of less than 1% of the diameter of surface 15 at its smallest cross section. In FIG. 5 these stabilizer-abutments are illustrated as forming part of the sliding cage 14, so that they move in unison therewith.

It is seen in FIG. 7 that for the direction of rotation indicated by an arrow, there exists a force component 33 pressing the member 9 against the part of the stabilizer 29 at the right side. This force 33 is the sum of the forces of rolling friction losses 31 and 32 which originate at the points 28 and 28' respectively. It is to be noted that the force 32 comprises the rolling friction losses at 28' plus the friction losses at the shaft 23 reduced to the circumference of the rotatable body 22. In the structure as illustrated in FIGS. 5 and 7, there is created by the pressure 33 against 29 a friction force 34 which opposes the fotation of member 9 and has to be added to the force 31. The force 34 could be further reduced by replacing the stationary (non-rotary) abutment-stabilizers 29, 29, by rotatable bodies. However, since the losses due to rolling friction at 28 and 28' are of an order of 0.003 to 0.005 P, the force 33 will be of the order of 0.008 P, or less, and the additional force 34 will be between 3% and 6% of force 33, or between 0.00009 P and 0.0003 P. Inclusion of the effects of sliding friction 34 at the contact of the member 9 with the stabilizer will alter the total force required at point 28 by between 3 and 1l /z%. This figure could be reduced to about between /2 and 2% by using rotating stabilizer abutments instead of the stationary type shown in FIG. 5.

FIGS. 8, 9 and 10 illustrate a structure which combines a resistance element 101, 102 (similar to the element 1, 2 of FIG. 3) with a rolling member 109 shaped to make contact with the wire windings on the inside of the ringshaped element 101, 102. The rolling member 109 has a substantially cylindrical form with a spherical end cap 117 on each end. The rolling contact between the wire winding and the element 109 takes place along the cylindrical portion 115. A pair of contact fingers 119, 119 touch the part-spherical endcaps 117 in the polar contact region 118 with a predetermined resilient force. The contact fingers 119, 119 form again part of a U-shaped collector member 120. The terminal for the contact arm is connected to the collector 120 by flexible wire similarly to the connection illustrated in FIGS. 3 and l. The contact arm 105 is supposed to have a pivot similar to FIGS. 1 and 2, so that it can swing inside the resistance element.

The rotating contact member 109 is resiliently forced into contact with the wire winding by the rolling element 122 which is freely rotatable upon the shaft 123. This shaft is supported by the cage 114 which in turn is slideable in the cavity 104 of the contact arm 105. A spring 125 provides the resilient pressure forcing the cage 114 radially outward to maintain a rolling contact between the surface 115 and the wire winding of the resistance element 102. The rotatable body 122, which transmits the spring pressure to the contact member 109, has a barrel shaped surface. Thus the spring pressure is transmitted from 122 to 115 substantially in the middle of 115, which is thereby capable of transmitting a substantially uniform pressure over the entire length of the contact between 115 and the surface of the wires 102. Frictional and other forces arising during the operation of this structure are very similar to those of FIG. 5 and the schematical diagram of FIG. 7 serves equally for the structure of FIGS. 8 and 9. Abutment stabilizers 129, 129 (FIG. 9) are provided at both sides of the cylindrical surface 115; due to the straight line feature the clearance between the surface of 115 and 129 can be held to less than /2% of the diameter of part 115. In this way a very high accuracy in maintaining an unchanging position of the contact area with respect to the position of the contact arm 105 is achieved. FIG. 10 which is a view from the right side of FIG. 8, with the resistance element 102 removed, shows that a line contact of substantial length is obtained between the abutment 129 and the cylinder surface 115, thereby greatly reducing any possible wear due to sliding friction at this contact. Further decrease of friction and possible wear is achieved by making the portion at 129 of the part 114 (FIG. 10) of a material of low coefficient of friction and great hardness, which may preferably be a non-conductor. Aluminum oxide in the form of sapphire has been found to have these desired qualities, particularly in combination with a rotatable contact member 109 made from a hard gold-platinum-silver-copper alloy known under the trade name of Ney-Oro. These alloys have physical and mechanical properties that are very suitable for sustaining small area contact loads under conditions accessible to predetermination by the methods of mathematical analysis of mechanical stresses developed by H. Hertz for the contact of elastic bodies and used for calculating stresses in ball bearings. I have found that resistivity, proportional limit, modulus of elasticity, ultimate tensile strength and Knoop hardness of the alloy mentioned are of sufficient grade to make it suitable for the construction of various forms of movable electrical contacts in accordance with the teachings of this invention. Application of the Hertz method will be more fully described in connection with FIGS. l2, 13, 14, 18, 21, and 22 to 30. It should be noted here that the rotating contact member 9 in all structures described in FIGS. 1 to 5 may be made of the Ney-Oro G alloy and the supporting rotatable bodies 22 (including 122) may be made of sapphire. Also, the contact fingers 19 are preferably made of the same alloy in order to avoid a moving contact between materials of different thermo-electrical potentials.

FIG. 11 illustrates a modification of the structure shown in FIGS. 8, 9, 10. The rotatable contact member has a cylindrical surface 115' which is provided with spherical end-caps 117'; however, in distinction to the previous structure of FIG. 10, these end-caps are now concave; they are each forming a spinning contact with the ends of the contact fingers 119' which are for this purpose pro vided with semi-spherical extensions 91' of predetermined curvature to assure the establishing of the proper contact geometry and physical stresses Within the contact area between 91' and 17'. Details of these conditions will be fully described in connection with FIGS. 22 and 23.

The conditions which control the contact geometry of a rolling contact, and thereby the mehanical and electrical stresses prevailing in the contact area are illustrated by the diagrams of FIGS. 12, 13, and 14. They represent partial views and sections of the rolling contact of FIG. 3 and enlarged about six times. The groove surface 15 of the rotatable member 9 makes contact with the surface of the resistance wires 2 which follow the same semi-circular contour of the upper edge of the resistance element. The geometric surface which envelops all of the wire surfaces at the semi-circular edge is one-half of a toroid whose radius of cross section is R and whose radius of the center of cross section is R The surface 15 of the member 9 is likewise a part of a toroid whose main axis is AA, its radius of cross section is R and its radius of the center of cross section is R +R These two surfaces have identical curvatures in a sectional plane containing the axis AA and the center and therefore they will make contact along an arc C C C This same are represents also the line of contact between the surface and an individual wire 2 provided that the center line of thewire lies entirely within the said sectional plane. If the plane containing the centerline of the wire forms a small angle with the sectional plane, the length of contact is shortened, the degree of shortening depending upon the angle between the plane of the wirecenterline and the sectional plane. This angle is hereafter referred to as the skewing angle. It is evident that the disclosed structure furnishes a means to select the length of the contact within a substantial range; this range has been found suflicient to pre-selected all pertinent dimensions of the contact so as to keep the mechanical and electrical stresses within limits for reliable continuous operation over long periods of time, by keeping the mechanical stresses within the fatigue limits of the selected materials.

This is one of the very important improvements over previously known structures, where the meeting of convex surfaces with inadequate contact geometry have resulted in practically uncontrolled and widely fluctuating conditions of mechanical stress often exceeding the limits of proportionality and yield stress. Metal fatigue, local embrittlement and irregular changes in electrical resistivity are but a few of the detrimental effects of these uncontrolled conditions. The almost instantaneous changes in load conditions which occur when a small-area ball contact switches from one wire to the next have the physical effect of a structural discontinuity, which in turn will cause changes in the electrochemical potential (or Fermi energy) in the lattice structure of the two materials entering into contact; these disturbances add to several others to form the cause for the electrical noise observed during the operation of most movable contacts.

In a rolling contact structure in accordance with this invention, the mechanical and electrical shock which accompanies the sudden transfer of load from one wire to the next, is eliminated by a geometry of design which gradually transfers the load from one wire to the next. This is obtained by introducing a skewing angle of special, predetermined size between the plane containing the axis AA and the plane containing the centerline and crestline of the contacted wire. The skewing angle has been mentioned in the last paragraph; also its effect to shorten the length of contact between the rolling member 9 and the wire. This is illustrated in FIG. 12 by indicating shorter zones, such as Z in the middle of the arc C -C C As the roller 9 moves over the wire winding the contact begins with a zone Z at the right side, as seen also in FIG. 13. This contact moves to Z as the roller moves downwards over the wires shown in FIG. 13, then to Z in the middle, further to Z and finally to Z at the left side. The skewing angle is so predetermined that the contact zone Z at the right side (FIG. 13) begins to form at the same time that the zone Z on the previous wire decreases. Please note that the travel of the contact zones from Z over Z and down to Z takes place on the surface of one wire, as shown in FIG. 12. The successive positions of the contact zone are shown on successive wires in FIG. 13 only for the purpose of clarity. Only the first contact zone on the last wire, Z is shown in its correct position to Z of the fore-last wire is FIG. 13. The dash-dotted line marked G in FIG. 13 is the trace of the plane containing the axis AA with the toroid surface that envelopes the Wires 2, in its position where the roller makes contact with the first wire (top) of FIG. 13. The next line, marked G is the trace of the same plane when it has moved down to a position where the member 9 or its toroidal surface 15, makes contact with one wire at each end of the arc C C C Since the electrical conductivity of the contacts is substantially depending on the size of the contact area, it is seen that the gradual transfer of the mechanical load carries with it a gradual transfer of the electrical load which is a very desirable factor in the reduction of contact noise.

In computing the conditions of electrical loads in materials subject to high mechanical stresses, as is the case in a rolling contact for the material close to the actual contact, it is necessary to consider the changes in resistivity which occur in the highly stressed strata. The resistivity of several metals drops to a fraction of its normal value when subject to high compression stresses of the order that may occur in ball bearings. Selecting a suitable contact area and computing its required special geometry for keeping the mechanical and electrical stresses within safe limits, is preferably done by using an adaptation of the basic formulae developed by Heinrich Hertz in his analysis of the contact between elastic bodies (Leipzig 1895, vol. 1, pp. l57l 82). Using the nomenclature of Hertz, the main curvatures of the first body are 6 and 6 for the second body the main curvatures are 6 and 6 the plane of 6 is perpendicular to the plane of 6 and the plane of 6 is perpendicular to that of 5 The angle between the plane of 6 and 6 is termed w.

For the analysis of conditions at point C (FIG. 12) the respective curvatures are:

for the body #1 (the rotatable member 9) l (convex curvature) l 6 (concave curvature) for the body #2 (the wire 2) The curvature of the wire winding remains practically constant over the entire length of the arc C -C -C while the curvature 6 for the body #1 changes from point to point along the arc. For the point C the curvature 5 (wire dia.= 12) while for the points C and C the curvature cos 7 s center upon the axis AA and touches the toroidal surface 15 in the point C It is seen that Similarly for other points between C and C the radius determining the curvature is the radius of rotation of the particular point divided by the cosine of the respective angle 7 for this point.

It is also seen from FIGS. 12 and 14 that the radius of curvature in the plane perpendicular to the axis of rotation A-A increases from the center at C to both sides thereof, so that it is at a maximum at the ends of the arc. This condition contributes to a reduction of stresses near its edges (i.e. near the points C and C where the transfer of load from one wire to the next takes place.

The contact area for any point along the arc C C C is an ellipse, having a major axis 2a and a minor axis 2b which are given by The auxiliary factors [1. and v are derived from formulae and tables given by H. Hertz, and which are also found in H. Stellrecht Belastbarkeit der Waelzlager Berlin 1928. The entities 8 and 6 are coefiicients of elasticity of the respective materials in contact, defined by The force P, is the total pressure upon the contact area in a direction perpendicular to the tangent plane at the center of contact area. For the point C this force P is identical with the force Q in the structure of FIGS. 4 and 6; for the structure of FIGS. and 7, it is identical with the force P. For points other than C the perpendicular contact pressure P changes with the angle 7 and has to be computed for each selected point. For the points C and C where during the load transfer for a very short time each of the points carries the same load, the amount of the perpendicular pressure for each point is 2 cos y The skewing angle between the section plane containing the axis A-A and the plane containing the center-line of the wire 2 is the auxiliary angle a: of the Hertz formula, which serves to compute the auxiliary parameters a and :1. Variations of the skewing angle cause changes in the shape of the contact ellipse, and this may be utilized to adjust the contact area for optimum mechanical and electrical stresses.

When applying the Hertz formulae to the structures of FIGS. 8, 9, 10 and 11, the area of contact between the cylinder 109 and the wire 102 (taking place at the inside of the resistance element) is computed by using for the figures of curvature the following:

1 radius of cylinder and for the wire surface 2 =FEM l radius of wire section It is to be noted that under these conditions and with a skewing angle of w=0 the contact area will be a narrow strip covering the full length of the cylinder surface; when the cylinder is then moved over the surface of the wire winding, it will alternate in making full line contact with one wire, and then in one instant make contact with the next wire over the full length, then breaking contact with the first 'wire, etc. The undesirable effects of this mode of operation, particularly as regards electrical noise, have already been pointed out. According to one of the aspects of this invention, a correct operation with a cylindrical roller is obtained by introducing a predetermined skew-angle between the cylinder axis and the plane containing the wire-center-line. This angle is so selected that the contact area extends only over a portion of the entire length of the cylinder surface, and travels from one end of the cylinder to the other as the cylinder moves over the surface of the wire windings, making gradual contact with the next wire, while gradually decreasing the contact area 'with the first wire. In this way a gradual transfer of electrical load from one wire to the next is obtained. From a purely mechanical point of view this may be compared to the gradual transfer of driving power from one tooth to the next in a gear transmission with helical gears.

The required skew angle may be obtained either by the special manner of winding the wire, so that the inner portion of the full winding, which is the only part entering into contact with the cylinder, is inclined at the full skew angle with respect to the geometrical axis XX of the resistance element (FIG. 2); or, the angle just mentioned may be made to represent only a part of the total skewing angle required, and the balance of the skewing angle is then obtained by introducing a corresponding inclination between the axis of rotation of the cylinder 115 and the axis XX of the resistance element.

In cases where the skew angle of the cylinder axis 115 is more than a very small figure, it may be necessary to give to the surface 115 a slight barrel shape instead of straight cylindrical as before; the reason being that the intersection of a radial plane containing the axis of rotation of 115 and the center of contact area, with the geometric surface which is tangent to the inside of the resistance element 102 is then not a straight line but part of an ellipse. Under these circumstances, a straight cylinder surface 115 would ride on its edges at both ends and not be able to make contact at its central portions. The same corrective effect may also be attained in combining a straight cylindrical surface 115 'with a core of the resistance element 101 that has a slightly barrel shaped inside, the crown of the inside being so dimensioned that the intersection of a radial plane containing the axis of rotation of the surface 115, with the tangent surface of the wires at the inside of the resistance element, is a straight line.

The correct numerical dimensions in each case are determined by known methods of 3 dimensional analytical geometry or by graphic methods of solid geometry.

FIG. 15 illustrates an elevated section of two rolling contacts operating on concentric tracks. FIG. 18 shows a top view of the concentric tracks 202 and 203 with the rolling contact members indicated in dotted lines. FIG.

16 is a partial section of FIG. along the plane marked MM shown in a third-angle projection. FIG. 17 is a similar section of FIG. 15 along the plane NN. The rotatable contact members, marked 209 in all figures, are shown as balls. They may be steel balls having a plated surface of a suitable metal, preferably a gold alloy; or they may be made entirely of an alloy of desirable qualities, such as the gold-alloy mentioned which is available under the trade name Ney-Oro G. The values for modulus of elasticity which have been given heretofore as 17.10 p.s.i. correspond to this alloy. For the operation of the device it is irrelevant whether the support 224 is stationary and the disc 30 rotates, or vice versa, provided that in the case of the rotating support 224 the rotational speed is not so high that centrifugal and gyro-forces must be taken into consideration. Similar to the design in FIG. 3, a carrier 214 is mounted slideable in a cavity of the support 224. There is a separate carrier for each of the two balls 209. (FIG. 15, the carrier for the ball on track 202 is marked 214; the carrier for the ball on track 203 is marked 314.) FIG. 15 and 16 show that a roller 222 is mounted rotatably upon a thin shaft which is held by the carrier 214. The roller 222 contacts the ball 209 and when the carrier is pressed downwards towards the track 202 by a spring (similar to FIG. 3, but not shown in FIGS. 15, 16) the spring pressure is through the roller 222 transmitted to the ball 209, making a rolling contact with the track 202..The track is formed by commutator segments 202 embedded in a disc of insulating material 30. A shallow groove 230 (FIG. 18) in which the ball 109 rolls, is provided with a curvature of cross section predetermined to produce in contact with the ball surface an area of contact 231, computed on the basis of stipulated physical stresses.

The width of the groove is made equal at least to the length of the major axis (2a) of the contact ellipse and preferably a little wider (FIG. 21). The major and minor axes of the contact ellipse are computed by use of the formulae referred to in connection with FIG. 14. The transmission of the electric current from the ball 209 to the other parts of the circuit is obtained according to this invention by spinning contacts between the ball surface and the contact fingers 219, 219; the contact fingers are shown as extensions of the U-shaped current collector 220. This part of the structure is basically the same as shown earlier in connection with FIGS. 3, 4 and 5. Similarly, the connection from the outer parts of the circuits to the collector 220 may be through a wire attached to the collector 220 (compare FIGS. 15 and 3). The support roller 222 is preferably made of insulating material and more particularly of sapphire or some similar substance which is hard enough to sustain the rolling action of the ball upon it and which does not require lubrication for its rotation upon the shaft. The kinematic of operating forces in FIG. 16 corresponds to the diagram given in FIG. 7. It is therefore seen that for the purpose of defining and stabilizing the position of the ball 209, separate means must be provided; these are illustrated in the form of the stabilizer abutments 229, 229, which extend from the carrier 214 downwards on both sides of the ball, in the direction of rotation; this stabilizer-abutment may also be made in the shape of a ring closely surrounding the ball near its equator, and having two cut-out portions where the contact fingers 219 extend from above to touch the surface of the ball 209, in polar spinning contacts, similar to 18 in FIGS. 3 and 5.

In the structure shown in FIG. 17, the ball 209 is supported by two rollers 322, 322, both being rotatably mounted in the slideable carrier 314; the kinematic force diagram of this structure corresponds to FIG. 6, and for the same reason as there described, no further means are required to define and stabilize the position of the ball 209. The combination of a two-roller support structure, such as FIG. 17, with a commutator track such as shown in FIG. 18, produce the most stable and consistently accurate position of the contact area between the ball 209 and the sectors of the track. Postulating practically obtainable tolerances for all parts, the variations of indexing of the instant of making (or breaking) of the contact between the ball and the respective sector edge (but not including pitch errors between successive sector edges) can be held to a range of Ab+t where b is one half of the minor axis of the contact ellipse and t the tolerance (or error) of the position of the ball center. For ball sizes substantially between 0.046" and 0.625", the error if can be held to between 0.0003 and 0.0005 inch. For a ball of 0.0625 dia. pressed by a force of 5 grams (0.011 lbs.) against a groove profile with a groove radius of 0.52 of the ball dia., the contact ellipse has a major axis of 2a1=2.58-10- inch and a minor axis of 2b-=0.318-10- inch. Since the making of electrical contact must take place between the edge of the contact area (pressure=zero) and the point of maximum pressure at the center, it can safely be assumed that making of contact has taken place not later than between a point where the stress is of the maximum and the center. This point of 85% specific pressure (or 85% of maximum stress) is located very nearly halfway between the edge and the center of the contact area. This point is 0.08 10- inch from the edge of the contact area so that the total range for the error in making electrical contact is then 0.0003 plus 0.00008, or a total of 0.00038 inch. This is a very substantial improvement over the performance of commutator devices with sliding brush contacts, where the limitations of minimum brush thickness, deflection, wear result in position errors for the point of making contact which are 8 to over 12 times greater than the above figure. It must also be noted that over 75% of the error range of the new device as above cited, was allocated to an estimated sum (0.0003") of the following individual tolerances: eccentricity of the support rollers 222 upon their shafts, sideways play between carrier 214 within the cavity of the support 224 (FIGS. 15, 17). The latter of these errors can be completely eliminated by suitable design, for example using an elastic linkage suspension for the carrier 214; the first of the named errors can be further reduced without excessive cost to 0.00002 inch. Thus the total error of the position of the point of making contact can be reduced to 0.0001.

In combination with an encoder disc of 1" effective dia., the rolling ball contact as here described will deliver an angular accuracy equivalent to an angle having a tangent of 0.0002; this corresponds to an angle of 41 sec. of arc. These figures do not of course include errors contributed through eccentricity of the commutator disc, or pitch errors of the segments because these errors will have the same additional effect with contact by sliding brushes.

The importance of providing a groove of predetermined profile for obtaining a contact area of stipulated dimensions and for controlling the maximum specific contact pressure, so as to keep max. stress below safe figures for the materials employed, will become apparent from the following figures:

A ball of 0.0625" dia. rolling upon a flat plate while carrying a load of 0.011 lbs. (5 grams) forms a circular contact area of 0.0006 dia. if both are of the same material with a modulus of elasticity of 17-10 p.s.i. The same ball with the same load rolling in a groove with a cross section radius of 0.52 of the ball dia., forms a contact ellipse with a major axis of 0.00258" and a minor axis of 0.000318". Since the minor axis is in the direction of motion (tangent to the track circle) the effective width of the contact area for the groove is nearly /2 of that for the flat plate. At the same time the contact area has increased from 0.282-10 sq. in. to 0.643-10- sq. in. The mean specific pressure has decreased from 39,000 p.s.i. to 17,000 p.s.i.; and the maximum spec. pressure,

13 which controls the load carrying capacity has decreased from 58,500 to 25,650 p.s.i.

Considering that the proportional limit for certain gold "alloys is around 70,000 psi, it appears that a stress figure of 58,500 psi would be too high for a load cycle repeating several millions of times, if premature fatigue failure is to be avoided. The greatly reduced stress figure of 25,650 psi. of the groove contact, due to properly selected contact-geometry, offers an assurance of a life period of at least 140 times the life of the highly stressed contact, based upon the experience of fatigue life in ball bearings.

Similar improvements in extending the useful life of the device are obtained in the case of potentiometers. Considering a wire wound resistance element (FIGS. 2, 3) having a wire of 0.002" dia., wound upon a core having a radius at its ends (R in FIG. 12) of 0.048" so that the radius of the wire surface is 0.50", and a rolling contact member 9 with a toroidal groove with a minimum diameter of 0.096" (i.e. R =O.048"), the maximum stress, depending on the predetermined length of the contact zones (Z etc., in FIG. 12) is between 28,500 and 40,000 p.s.i. A conventional design, using the same resistance element with a ball of 0.200" dia. will under the same contact load of 0.011 lbs. generate a maximum stress of 187,500 p.s.i. Since the proportional limit for the best available alloy with heat treatment is not over 140,000 p.s.i. it is evident that under such stresses permanent deformation of the resistance wire takes place, resulting in alteration of its cross section at the contact points. Due to the very high stresses, the resistivity is subject to alterations and the original linearity of the potentiometer is impaired.

A modification of the structure of FIGS. 15 and 16 is shown in FIGS. 19 and 20 where the support roller 222 is replaced by a bearing pad 414 which is slideable in a suitable cavity of the plate 424. A spring 421 supplies the pressure required to maintain electrical contact between the rolling member (or ball) 409 and the track 402. The electrical current is transmitted to the ball 409 through the contact fingers 419 which make spinning contact at the poles of rotation in the manner described in FIGS. 15 and 16. It is seen that the structure of FIG. 19 is in certain respects similar to that of FIG. 16, with the rolling support body 222 having been replaced by the bearing pad 414. In its simplest form, that is, without providing a special geometric shape for the bearing pad 414, this arrangement is practically inoperative because the amount of uncontrolled sliding friction that arises between the surface of the ball 409 and the surface of the bearing pad 414 prevents the ball from reaching a state of synchronous free rotation. By synchronous free rotation is meant that the ball will have, and maintain a rotational speed at exactly the figure which corresponds to the difference of motion between the track and the ball surfaces, taking into account the difference between the diameters of the respective pitch circles; for a rolling contact whose contact area is a mathematical point, the two pitch circles meet at that point, and the ratio of the roaional speeds of the two bodies is the ratio of the two pitch-circle diameters. In the case of a contact area of finite size, it has been found that when tractive force is transmitted through the rolling contact area, the point where the two pitch circles meet (and which determines the speed ratio by its radii to the two individual axes of rotation) need not be the geometric center of the contact area. Stable transmission of force with a finite ratio of speed between the two bodies is obtained as long as the meeting point of the pitch circles is within the contact area. The term free synchronous speed of rotation includes therefore all values of speed for the driven body for which both pitch circles meet within the contact area.

Up to the present, known devices attempting to use balls as rolling contact members have produced only highly unsatisfactory results; investigation has shown that due to basic errors and deficiencies of design andconception, free rotation of the ball (as hereabove defined), was not-and neither could have been-obtained; it also indicates the failure of previous attempts to recognize the true nature of a rolling contact as a miniature friction transmission by rolling tractive contact. Extensive experiments with rolling traction between bodies of hard material has shown that reducing the rotational speed of one of the bodies below the indicated limits will start a chain of deleterious effects leading to the destructon of the contacting surfaces. These effects, as far as they concern electrical rolling contacts, will be more fully described on hand of FIGS. 29 and 30.

According to this invention the required free rotation of the contact ball is obtained by balancing within predetermined limits the tractive forces and the resistive forces which act upon the ball or other rotatable contact member. Recognizing the fact that rolling contact devices are basically miniature friction transmissions leads to a clear understanding of the operating forces involved. It is seen that in all of the disclosed structures the rotatable contact element (9 or 109, or 209, etc.) is caused to rotate only through the transmission thereto of tractive forces in the rolling contact area between the rotatable body and the track (2, 11, 102, 202, etc.); this holds true whether the cage carrying the contact ball or roller moves rela tively to a stationary track, or vice versa.

The structures of FIGS. 19 and 20 show the ball 409 cradled in a concave recess of the bearing pad 414; the surface of the pad is shown schematically to be identical with the ball surface. This is approximation only of the specific conditions as required for this surface by this invention. It has been found that the bearing pad 414 must be made of a material incorporating specified physical properties and that the geometrical shape of the bearing surface must meet specific predetermined requirements in order to obtain an operative device according to this invention. Flat bearing pad surfaces which have been previously proposed, either singly or in V-paired shape, are ruled out by the here specified requirements because the high stresses which arise at the center of the contact of the ball with these flat surfaces far exceed the limits tolerable for a sliding bearing support. The frictional losses which ensue from the combination of these stresses with the peripheral sliding speed of the ball surface cause a rapid deterioration of the sliding surfaces to the point of interfering with the free rotation of the ball. Various other shapes of bearing supports for rotating contact balls, such as cone-shaped sockets, or ring-shaped toroidal surfaces, are totally unsuited for carrying out the object of this invention, because the forces tending to return the ball to its position of equilibrium are either constant (as for cone-sockets) or have a negative characteristic, either of which prevents the ball from finding a point of stable dynamic equilibrium. With a cone-socket, the ball will either stay at its position or static equilibrium, in which case the friction forces will prevent its rotation; or the minutest increase in the tractive forces upon the ball by the rolling contact (at the point C will yank the ball out of the socket. In the case of a toroidal-ring support, the conditions are even worse because the stabilizing forces have a negative characteristic beyond a certain point.

FIGS. 26 and 27 illustrate the condtions for establishing stable dynamic equilibrium and free synchronous rotation of the ball in accordance with this invention. FIG. 27 which is an enlarged partial cross section shows the ball in two positions: 409' is the position of static equilibrium, and 409 which is the position of dynamic equilibrium, 414 is a section through the bearing pad showing the spherical concave surface S having a radius 1' and a geometric center 0 C is the point of contact (more correctly: the center of the contact area) for the position of static equilibrium 409' of the ball. C is thepoint of contact for the position of dynamic equilibrium 15 409 of the ball. To simplify the diagram, it is postulated that the bearing pad 414 is stationary, and that the track 402 is pushed vertically upwards, in addition to its surface motion in the direction of the arrow V In reality, the track stays always on the same level S having only the motion V while the pad 414 presses resiliently downwards. To represent these actual conditions would confuse the drawing by requiring to show two profiles S spaced apart by the amount now shown between S and S and to duplicate the center together with the radii to the points C and C The diagram as shown illustrates the lateral displacement of the ball as it changes its position from static to dynamic equilibrium; the lateral displacement of the ball center, marked L is found from the triangle O '-O -O to be L: (r -r) -sin alpha The quantity L also denotes the displacement of the contact area (ellipse) between the ball and the track 402, as shown in FIG. 28. Inspection of FIG. 27 shows that the ball would occupy the position 409' if it is subject only to the vertical upwards thrust P of the track 402 and provided that the contact of the ball with the track does not transmit to the ball any tractive force (frictionless contact), or if the track, while being pressed against the ball will follow any lateral motion of the ball freely. As soon as a tractive force is transmitted from the track 402 to the ball in the direction of V the ball starts rotating as indicated by the arrow D and it begins to climb up on the curved incline of the surface S the contact point C moves up to C and the ball center moves from O to 0 It is postulated that for the point C the friction force between the ball and S (which provides the traction for the ball to climb the incline S is slightly over-balanced by the tractive force transmitted to the ball at C so that the ball surface slips, or glides against the surface S at the point C while rotating about its center at 0 This is the position of stable dynamic equilibrium for the ball, because if the ball is displaced from it by a transient unbalance of forces, it will return to this position upon restoration of the balance of forces. The relation of these forces to the friction coeflicients in the points C and C and to the geometry of the parts is shown diagrammatically in FIG. 26. The ball 409 is assumed to be in the position of dynamic equilibrium between the track S which is driving it through the tractive contact at C and the plane surface S which is the tangent plane to the ball in the point C which corresponds exactly to the point C in FIG. 27. The plane S is therefore also a tangent plane to the surface S in the point C FIG. 27. The pressure P is perpendicular to the track surface S and forms an angle 90-alpha with the inclined tangent plane S The tractive force acting upon the ball in point C is H and its maximum value is H1=P1',LL1, where ,u. is the friction (or traction) coefficient in the rolling contact C This force causes rotation of the ball in the direction of the arrow D. In the contact at point C the ball surface is sliding relatively to the surface of the tangent plane S and produces a friction force F given as where P is the pressure in point C perpendicular to the plane S and directed towards the center of the ball. P

is the resultant of the two forces P and H acting upon the contact between the ball and S in the point C It is seen that H =P tan alpha and since both horizontal components must balance each other, it follows that P -,u. =P -tan alpha and p =tan alpha In order to assure rotation of the ball, with a tractive non-slip rolling contact at C and full peripheral speed at P2 cos alpha The relation takes into account only the traction between the ball surface and the track surface on the one side and the friction between the ball and the bearing pad surface on the other side. It shows clearly that in the case of practially equal friction coefiicients at both points, the ball can not rotate, even if the angle alpha were zero. Since none of the previously proposed structures using balls as rotatable contact elements made any provisions to cope with this condition, these devices were inoperative.

According to the present invention, the necessary friction-ditferential is obtained substantially by two measures. The first measure is to give a special shape to the bearing pad surface, which is so formed that it produces a minimum of specific pressure in the contact area between the ball and the pad, while at the same time providing for the ball a small amount of lateral freedom of motion within the predetermined limits of permissible lateral displacement; the second measure is the selection of a special material for the bearing pad surface which should be non-conductive, of great surface hardness of not less than 7 and preferably 8-9 on the Mohs scale, and capable of maintaining a high grade surface finish of better than 5 microinches A wavelength). An example of such material is aluminum oxide in the form of sapphire.

It has been pointed out that the bearing surface of the pad must have a concave profile (or cross section) at least in the direction of the motion of the track; it should be noted that the profile need not necessarily be a circle, but could be a concave portion of another curve of the second or a higher order. In combination with a ball as rotating contact element, it is preferable to use a spherical bearing surface for the pad because of practical considerations of manufacture. Therefore, the concave pad surface in FIG. 27 is spherical (S and its radius r is shown in an exaggerated ratio to the ball radius r. As a result of extensive research and prolonged durability test runs it has been found that the radius of curvature r should be between substantially 0.5% and 6.4% greater than the radius of the ball. A preferred figure is between A: and 2 /2 With a ratio of 0.8% and a ball diameter of 0.0625", under a pressure of 0.11 lb. (5 grams) the specific pressure between the ball surface and the pad surface is between 3900 and 8000 p.s.i., so that a reliable sliding bearing operation with low friction coeflicient is obtained. This specific pressure is less than 11% of the specific pressure which occurs in the contact of the same ball with a fiat plate, under the same total load of 0.011 lb. (58,500 psi). It is evident that under such pressures and at the velocity of the ball periphery, serious damage to the sliding surfaces must be expected and this made previous attempts unsuccessful.

The ratio of pad surface curvature to ball radius must be held substantially within the stated limits in order to avoid vibrations of the ball about its position of equilibrium; such vibrations usually start as lateral vibrations which then develop vertical components. While the amplitude of such vertical vibrations may be so small that it escapes observations, the resultant variation of the effective contact pressure leads to increased electrical noise and also causes a permanent modulation of the track surface with a minute wave pattern, that destroys the smooth surface of a perfect track for the ball. These modulations are so small that inspection with a conventional metallurgical microscope with vertical illuminator will not reveal their existence. Microscopes with special illuminating devices, such as the Ultraphot of Carl Zeiss with its oblique illumination system are required to show clearly the difference between a really smooth track surface, and one with vibration marks. This is illustrated in FIGS. 29 and 30. Both are fairly accurate reproductions of photomicrographs made at 160 diameters with oblique illumination system on the Zeiss Ultraphot. A scale marked in 0.001 has been added to permit an estimate of the true dimensions. The ball had a diameter of 0.0625" and the pressure was 4.5 grams. The indentations made by the oscillations of the ball, as seen in FIG. 30, show a remarkable regularity, presenting a wavelength of very near 0.005"; the oblique illumination reveals clearly the concavity of the indentations. Assuming that the profile of a radial section is nearly identical with the curvature of the ball surface that caused it, the maximum width of the indentations would represent the chord of the arc; measured at about 3.5.10- inch, this gives for a ball of 0.0625 dia. a subtended angle of 25=2 312'. The height of the segment, which is the max. depth of the indentation is then r-(1-cos 6)=0.0000487" (48.7 microinches). The frequency of vibration is found as the number of indentations passing per second. The total track length is 2.98 which represents 595 waves (or cycles). At a rotational speed of 3.68 rev./ sec. of the track, the frequency is 2190 cycles/ sec. While no noise corresponding to this frequency could be observed, it was clearly visible (with a microscope of 50X) that the ball rotated at a speed substantially lower than even half of its synchronous free speed which should have been 55.8 rev./sec. This observation indicates that a great amount of slipping, or sliding, took place at the contact C which under these conditions could not be expected to show a satisfactory performance. The comparison of the track of FIG. 30* with the perfectly smooth track of FIG. 29, obtained under equal conditions of testing time, load and speed, show clearly the basic improvement achieved by a true rolling contact in accordance with the teachings of this invention.

In discussing FIGS. 26 and 27, only one of the torques which oppose the rotation of the ball was considered, the friction torque T originating at the point C There is however another torque which opposes the driving torque T and this is the friction torque T caused by the friction of the contact fingers which serve to transmit the current from the ball (or other rotating contact element) to the other parts of the circuit. These fingers are denoted 19 in FIGS. 3, 4, 5, 12; also 119 in FIGS. 8, 9, 10; 219 in FIGS. 15, 16, 17; 419 in FIGS. 19, 20; and 19 in FIG. 22.

The peculiar kind of contact which exists when two conductor surfaces meet in a common contact area while one of the conductors rotates relatively to the other about the geometric axis through the center of the contact area, has ben previously designated in this specification by the special term spinning contact, to distinguish it from a conventional sliding contact, or a rolling contact. FIG. 22 shows a greatly enlarged section of the contact between a spherical surface 17 with a radius of d/2=0.03125" and a flat plate 19; the center of the circular contact area being the original point of contact 18. All dimensions are shown in correct relation to each other. On the basis of a total pressure (or load) upon the contact area of 0.011 lbs. grams) and a modulus of elasticity for the material of both parts of 17.10 p.s.i., the circular contact area has a radius r =0.3.10 inch. The specific pressure is a maximum at the center, 58,500 p.s.i., and decreases to zero at the periphery. The distribution of the changing specific pressures between these two extremes follows a semicircular diagram, as shown. The equivalent mean pressure, which, uniformly distributed over the entire area, would give the same total pressure or load, is denoted H, and its value is of the maximum pressure at the center. For the purpose of explaining some of the peculiar properties of this type of movable contact, the entire contact area has been divided (arbitrarily) into 6 ring-zones (see FIG. 23) and the specific pressure which exists at the inner edge of each ring zone is shown in FIG. 22. The linear sliding velocity between the two contacting surfaces is a maximum at the outer edge of the contact area (radius r and decreases to zero at the center (point 18); the straight line V gives the relative linear sliding speed for each of the ring zones. An approximation of the friction torque of this contact can be found by computing the effective pressure for each of the ring zones and multiplying it with'the respective mean radius for each zone and with the coefiicient of friction. A correct value is found by the integration of infinitely small ring zones as T3 'P./L -k-r where P is the total pressure or load; ,u the friction coefiicient; r the radius of the contact periphery; k is a factor obtained by integration so that k.r is the radius at which the entire load P has to be concentrated to give the same friction torque as the distributed load. The value of k is 0.6325, so that T =0.6325.P.a .r

Assuming for the value of the friction coefiicient u the same value as for the driving contact in C that is the friction torque for a spinning contact between a ball of 0.0625 dia. and a fiat plate under a load of 5 grams (0.011 lbs.) is found as T =2.095.10- in lbs.

With ,u =0.l, the friction torque T =2.095.10 in lbs. For the purpose of comparison with a sliding (brush type) contact using the same pressure upo na contact sliding at the circumference of the ball, it is found that the friction torque will be T P2./L1 and for -=0.1

T =3.44.l0- in lbs.

It is seen that the introduction of the spinning contact has reduced the friction torque (due to the current-pickup from the rotating contact element) in the ratio of 1.

In the case where two spinning contacts are used on opposite ends of a ball diameter as illustrated in FIGS. 3, 15, 19, or on opposite ends of the axis of rotation of a rotating contact element, as illustrated in FIGS. 8, 10, 11, and where the sum of the two contact pressures is P, there is a further reduction of the friction torque because of a reduction of the size of the contact area for each of the two contacts; the radius r of the contact area under a load of /2 P is lherefore, the friction torque 1 for both spinning contacts will be T I -.7 T =1.665.10 111 lbs.

Compared to the friction torque caused by a sliding peripheral contact with the same friction coefficient, this is a reduction in the ratio of 206:1. It is evident from these figures that the friction losses caused by the current collecting fingers or brushes of previous designs have by the new structure of this invention been reduced to such a small fraction of the tractive forces available from the rolling contact at C that in a first consideration of the relations between the forces at S and C necessary to assure free synchronous rotation of the ball, the elfect of the friction by the current collecting fingers can be discharged in order to simplify the analysis.

The total loss of energy caused by the transfer of current from the rotatable contact elements (9, 119, etc.) to the contact fingers (19, 119, etc.) comprises the losses by mechanical friction and by electrical resistance in the contact itself and its immediately adjacent conductive material. For the example here considered, with a ball of 0.0625" dia. rolling at free synchronous speed upon a track of 2.98" circumference and rotating at 220 r.p.m., the loss of mechanical energy for the two spinning contacts positioned at opposite ends of the rotating contact element is This is 0.48% of the total traction energy transmitted at C which is E =T .w=344.lO* .35O=l2.05.10 in lbs/sec. or

1.36.10- watt The electrical losses caused by a spinning contact are likewise comparatively small in spite of the high current density in the contact area proper; the reason for this lies in the very short current path through the area of very small cross section. FIG. 22 shows in the lower half of the drawing in dotted lines a system of curves illustrating an approximate current path for each of the six ring zones of the contact area. It must be pointed out that calculation of the electrical losess can furnish only an approximate figure, because the distribution of current density over the contact area and the conductive material immediately adjacent thereto changes with variations of resistivity along each current path. Such variation may be caused by mechanical stresses and localized heat effects. It is known that certain metals show a marked decrease of resistivity under high compression stress. An approximate value of electrical energy loss can be obtained by plotting the path of the equivalent mean current through the contact and adjacent conductive material, and computing the resistance using resistivity figures; the final result may be somewhat on the high side.

Analysis of FIG. 22 shows that the length of the mean current path through the region of high current density is substantially represented by the length of a 70 arc with a radius of 0.785 of the radius of the contact circle,- (r Using as a resistivity figure for the gold alloy considered as material for the contact parts a value of 80 ohm cmf., the resistance for one contact under a 2.5 gram load is the voltage drop at 2.5 ma. current is v.=13.2.10 volt and the energy loss for both contacts ma. 13.2 volt) It is to be noted that in computing the current distribution and current path (or flow lines) the two conductor bodies, 9 and 19 in FIG. 22, have been treated as if they represented one piece of material; this is made possible by a very important and unique quality of a spinning contact, which is, to behave like a contact between two stationary conductors while actually one of them is rotating relatively to the other. Here lies a fundamental difference between the spinning contact and any other type of movable contact (excluding liquid contacts). The flow of electrons through the conducting material on both sides of the contact area proper is in a steady state, not changing intensity or location, and the surface areas which touch each other in the contact never change. In both surfaces, each minutest portion of the contact area always carries the same amount of current (or electron flow). In a sliding brush type contact, there takes places a continuons covering and un-covering of a part of the surface of at least one of the two conductors as the contact surface of the brush sweeps over the other conductor. It is known that a free surface of a metal represents a discontinuity of the crystal lattice and that this results in a polarization layer, or double charge layer, at the surface of the metal. In a sliding brush contact, these conditions are not stationary but change continuously, the two surfaces join each other at the incoming side of the contact and they again separate at the outgoing side. Also, each atom in the lattice goes through a pulsed electron flow, from zero through a maximum back to zero. The cycle is mostly short and with a sliding brush its curve (or function) is uncertain, because the pressure distribution across the contact area of a brush is varying and uncertain. These basic differences are illustrated in FIGS. 23, 24 and 25. In FIG. 23 a portion of the ring zones are divided by radial lines (dotted) into sector shaped elements marked A, B, C, D, E. While they are shown as sectors of substantial dimensions they are to be thought of as very small or narrow conduits of the current (or electron flow). In FIG. 24 the outer row (of FIG. 23) of these conduits are shown as seen by an eye positioned in the plane of the contact circle (r The line Z which represents the geometric plane of the contact area separates the conduits of the conductor 19 (above Z) from the conduits of the conductor 9 (below the line Z). It is seen that the current flowing through the conduit A continues through the conduit A the current flowing through the conduit B continues through the conduit B and so on with all other conduits. It is to be understood that while current flow is illustrated in the conduits A -A B B C -C D D E E only, there is identical current flow through all of the conduits of each of the six ring-zones. If the conductor 9 starts moving relatively to conductor 19 in the direction of the arrow X, it is seen that the current flow of E will continue partly in E and partly in D and after further motion fill D completely, while E serves as conduit for the flow that was to the right of E before the motion started. All the other flows will change over from one conduit to its neighbor gradually. Since the intensity of flow is uniform and the same for all conduits in a ring zone (the intensity may vary from ring to ring) there is no change of current intensity involved in the sideways shifting of the current flow due to the motion of the conductors; consequently there is no cause for changes in magnetic fields, nor a change in localized heat development. The crystal lattice of both conductors moves laterally at a very slow speed through a uniformly distributed and steady field of electron flow. This is very different from what is happening in a brush sliding contact, as illustrated in FIG. 25. While the current flow in the brush is practically uniform and steady (except very near the contact area proper where the current density may shift rapidly between points of better and lesser contact resistance, due to non-uniform scoring within the contact area), the flow through the conduits of the conductor below the line Z A B C D E is a pulsed flow as far as the individual conduits are concerned. An empty conduit is suddenly filled with current flow whenever the brush sweeps over it, and it is emptied with the same suddenness. These changes in current distribution and intensity cause variations in magnetic fields, thermoelectric potentials, surface polarization layers etc. All these disturbances, which are contributing to electrical contact noise, are prevented from occurring in the spinning contact due to the non-varying and uniformly distributed density of current flow along the entire path of relative motion for any point in the contact area.

that the surface portions on both sides, which establish the contact when they touch each other, remain in contact once the contact has taken place, whether the contact is in motion or standing still. In this way the critical surfaces are always protected fro-m influences by the am A further advantage of the spinning contact is the fact 21 bient atmosphere or other chemical or electro-chemical processes which may be accompanying the joining and separating of current conducting metallic surfaces.

It is understood that while the foregoing specification and drawings are given as illustrative examples of preferred forms of embodiments of this invention, there are many other forms of structures which are suitable for carrying out the present invention without departing from the spirit thereof, and that the disclosure herein is not intended to limit the scope of my invention and its application to the examples given. The structures of FIGS. to 21 show a rolling ball contact applied to a commutator where the segments have a groove of circular arc profile in order to obtain the required contact area without exceeding the specified physical stress limits. This novel means for reducing surface breakdown and wear is shown combined With another new structure for reducing mechanical and electrical losses in the current transfer from the ball, termed a spinning contact, which adds to the vast improvement of the first measure. Subordinated to these main objects is a special design of the contact fingers as parts of a U-shaped or forked member which is free to follow lateral movements of the ball in order to assure equalization of the pressures of the two spinning contacts. The anti-friction support of the rotating contact element, either through separately rotating support rollers, as shown in FIGS. 3-11 inc1., and 15 to 19 incl., or through sliding bearing pads of special shape such as shown in FIGS. 19, 20 and 26 to 28 incl., are further important contributions to the main object of the invention, to create a movable electrical contact with superior performance in both the mechanical and electrical field.

It is understood that the several aspects of the invention may be combined differently from the manner illustrated, and that, depending on special requirements of the particular case, other combinations may be found useful, and all such embodiments of the several objects of this invention shall be considered within its scope.

What I claim is:

1. In a potentiometer, a resistance element and a contact device movable relatively thereto along a predetermined path for varying the position of the point of contact therewith, said contact device including a rotatable conductive element having a surface of revolution about a geometric axis of rotation, one portion of said surface adapted to form a rolling electrical contact with a part of the surface of said resistance element, means operatively connected to said rotatable conductive element for movably supporting it and adapted to transmit a controlled amount of pressure through said rotatable element to the area of said rolling contact, the contacting portions of the respective surfaces of said rotatable conductive element and of said resistance element being mutually shaped to provide an elongated area of contact therebetween to reduce the specific pressure in said contact area to a magnitude within a predetermined range, said contact area being elongated in a direction parallel to said geometrical axis of rotation, and contact means engaging the surface of revolution of said rotatable element at a polar region thereof to form a spinning contact for transmitting electric current between said contact means and said rotatable element.

2. In a potentiometer as in claim 1, wherein said elongated contact area is skewed with respect to said predetermined path.

3. In a potentiometer, a resistance element and a contact device movable relatively thereto for varying the position of the point of contact with said resistance element, said contact device including a rotatable conductive element having a surface of revolution about its rotational axis, said resistance element having a portion of its surface adapted to form a rolling contact with said conductive element, means for rotatably supporting said conductive element and for holding it in rolling contact with said surface of said resistance element, means for maintaining the required predetermined amount of pressure upon said rolling contact between the mating surfaces of said resistance element and said rotatable conductive element, the curvature of said mating surfaces being made conforming to the particular shape which is for reasons of contact pressure, physical constants of the material of the mating elements and permissible stress-cycle fatigue limits required by the known formulae on the contact between elastic bodies by H. Hertz.

4. In a device for establishing rolling electrical contact between relatively movable conducting members, the cornbination comprising: a first conductive element and a contact device movable relatively thereto along a predetermined path for varying the position of the point of contact therewith, said contact device including a ro tatable second conductive element having a surface of revolution about a geometric axis of rotation, one portion of said surface adapted to form a rolling electrical contact with a part of the surface of said first conductive element, means operatively connected to said rotatable second conductive element for movably supporting it and adapted to transmit a controlled amount of pressure through said rotatable second conductive element to the area of said rolling contact, the contacting portions of the respective surfaces of said rotatable second conductive element and of said first conductive element being mutually shaped to provide an elongated area of contact therebetween to reduce the specific pressure in said contact area to a magnitude within a predetermined range, said contact area being elongated in a direction parallel to said geometrical axis of rotation, and contact means engaging said rotatable second conductive element at said geometric axis of rotation to form contact for transmitting electric current between said contact means and said rotatable element.

5. In a device for establishing rolling electrical contact between relatively movable conducting members as in claim 4, wherein said elongated contact area is skewed with respect to said predetermined path.

References Cited UNITED STATES PATENTS 1,053,219 2/1913 Rhodus 338-157 2,309,798 2/1943 Stoekle et a1. 338-157 2,524,199 10/1950 Lawrence 338-141 X 2,537,671 l/ll Jack et a1 338-157 X 3,219,960 11/1965 Volkmann 338-157 3,278,715 10/1966 Arbories 200-166 2,595,189 4/1952 Dewan 338-202 FOREIGN PATENTS 582,476 11/ 1946 Great Britain.

JOSEPH V. TRUHE, Primary Examiner I. G. SMITH, Assistant Examiner US. Cl. X.R. 338-157 

